1. Field of the Invention
The present invention relates to a radiotherapy system. More particularly, this invention is concerned with a radiotherapy system that includes a radiotherapy planning CT system having an X-ray CT scanner and capable of achieving a whole procedure ranging from imaging of a subject to radiotherapy planning based on the images, and a radiotherapy apparatus for irradiating a subject to accomplish radiotherapy, and that is effective for treatment of carcinoma or the like. The present invention is also concerned with a radiotherapy planner for determining radiotherapy planning data, which composes part of a radiotherapy system.
2. Description of the Related Art
In the past, radiotherapy in which a lesion such as a carcinoma is irradiate has been put to clinical use. The usefulness has been highly appreciated.
A linear accelerator is generally used as a radiotherapy apparatus for accomplishing radiotherapy. The linear accelerator generates radiation (X-rays) and the radiation by shooting an accelerated electron beam at a target a lesion of a patient lying down on a patient couch.
For treatments using the radiotherapy apparatus, various preparations must be made in advance. In the first stage, an X-ray CT scanner or the like is used to acquire images of a lesion. In the second stage, the images are used to accurately measure the location, size and contour of each lesion and the number of lesions. It is then determined what are the position of an isocenter, a dose distribution, and irradiation parameters (radiation field, angle, number of ports) enabling accurate irradiation of a lesion alone. In the third stage, an X-ray simulator is used to finalize an isocenter, position a patient through fluoroscopy, and draw markers on a body surface (isocenter markers or radiation field markers) on the basis of the determined position of an isocenter, dose distribution and irradiation parameters, and then performs simulation using the determined irradiation parameters.
When simulation is completed, after a certain period of time elapsed, treatment is commenced using the radiotherapy apparatus. Prior to the treatment, verification-oriented fluoroscopic images are used to verify the radiation field. The isocenter markers of all the markers drawn on the patient's body surface are used to position the patient, and then the radiation field markers are used to define the opening of a collimator. Thereafter, radiotherapy is actually carried out using the determined irradiation parameters.
In recent years, various approaches have been made to treatment of carcinoma. The significance of radiotherapy has been reassessed as a means for radical or palliative treatment. There is an increasing demand for more accurate locating of a lesion, more elaborate therapy planning, and more precise therapy.
Under these circumstances, a conventionally-known radiotherapy system comprises an X-ray CT scanner for producing tomographic images or scanograms, a radiotherapy planner used by an operator for making a therapy plan suitable for a lesion on the basis of the images, an X-ray simulator for use in positioning a patient on the assumption of actual treatment according to the plan data, and a radiotherapy apparatus for actually undertaking treatment. In integrated systems, the X-ray CT scanner and X-ray simulator of the four apparatuses share the same patient couch. Proposals for a radiotherapy planning procedure or a positioning procedure have been made in, for example, Japanese Patent Laid-Open No. 3-26278 (entitled "A method of controlling a positioning apparatus for radiotherapy planning") and Japanese Patent Laid-Open No. 3-224547 (entitled "A method of setting irradiation parameters for a CT scanner").
In the positioning apparatus described in the Japanese Patent Laid-Open No. 3-26278, three pointers (mounted in arm modules) for projecting light beams as an isocenter mark on the body surface of a subject are disposed at the upper part (ceiling), and right- and left-hand walls of an examination room. The arm modules are individually actuated using a driver. A misalignment between projected light beams emanating from the pointers on the right- and left-hand walls due to the mechanical strains of the pointers is compensated for in order to match the positions of the cross marks provided by the right and left pointers.
In the method of setting irradiation parameters described in the Japanese Patent Laid-Open No. 3-224547, a tomographic image rendering a target region of irradiation of a patient concerned, a multiplane reconstruction (MPR) image formed from the tomographic image with respect to a cross section containing a body axis parallel to an irradiation direction, and an MPR image rendering a cross section with respect to a body axis perpendicular to the irradiation direction are displayed in a screen of a display unit, so that irradiation parameters can be determined with the help of the displayed images showing the relationships between the target region and adjacent tissues.
As seen from the aforesaid prior art, when pointers for projecting crosslight marks are disposed on the walls and ceiling of a room, it is conceivable that the reference position of the positioning apparatus for projecting marks relative to a building may be displaced due to influences such as disturbances including indoor and outdoor vibrations, which are negligible but propagated to the building in the long run, and earthquakes. When such a displacement has occurred, the accuracy of marking an isocenter deteriorates. Every time the site of installing the positioning apparatus is changed, the positions of the pointers must be altered. Moreover, it is required to align the positioning apparatus and pointers. Uncountable labor and time must be consumed for maintenance.
As for the aforesaid procedure of setting irradiation parameters while displaying a tomographic image and two kinds of MPR images at the same time, the MPR images render parallel and vertical planes passing through a target region that is a lesion. For defining a radiation field suitable for the three-dimensional lesion, since the tomographic image does not necessarily provide a projection image rendering the target region with the longest contour, the position of a vertical plane cannot always be specified accurately. The MPR images or especially MPR image rendering the vertical plane must therefore be displayed and observed on a trial-and-error basis. This is a nuisance. It takes too much time for planning. However, when the operational procedure is simplified, the accuracy in setting irradiation parameters may deteriorate.
Aside from the aforesaid method of setting irradiation parameters using a tomographic image and MPR images, a method using axial (tomographic) images and a scanogram (fluoroscopic image) is also well known. However, this method has a drawback that since the images do not render a plane opposed to a certain irradiation angle of radiation, the opening of a collimator cannot be defined accurately for each irradiation direction during rotation irradiation. Because of this drawback and others, the method cannot keep up with the recent trend toward higher-precision therapy planning.
In any of the aforesaid methods of setting irradiation parameters, when a radiation field is defined and the beam fan lines of a radiation path is checked in an axial image, even if the positions of the beam fan lines are altered, the contour or size of the radiation field does not change. When the positions of the beam fan lines are found inappropriate, another radiation field must be specified by restarting the procedure from the beginning.
Further, a scanogram (a transmission image reconstructed from an X-ray image or a CT image) of a subject and reconstructed axial images (CT images) are usually used for therapy planning. That is to say, the scanogram or something equivalent is used to define a radiation field covering a lesion (target volume), and the axial images are used to identify beam-fan lines. The axial images are also used for plotting energy distribution on a slice.
The foregoing prior art has difficulty in coping with an ongoing demand for higher-precision therapy planning because of the drawbacks described below.
For example, when axial images are used to delineate beam-fan lines, as shown in FIG. 1, although a radiation path vertically transverses an axial plane (that is, an axial image) lying immediately below a radiation source, another radiation path T pierces an axial plane (axial image) PL.sub.AX lying away from an isocenter along a body axis. Thus, depending on the location of an axial plane, beam-fan lines (border lines of a radiation path) may not be delineated. Even if the beam-fan lines are delineated, the trajectory of the delineated beam fan is often hard to understand.
In conventional therapy planning, even if beam-fan lines can be identified in an axial image, a transmission image (scanogram or X-ray image) must be used to correct the contour of a radiation field. The beam-fan lines cannot be delineated in any image other than an image rendering a plane (axial plane) perpendicular to a longitudinal axis. Screens must therefore be changed depending on types of images. This is quite inconvenient, deteriorating operational efficiency.
For defining a radiation field, a scanogram or an X-ray image is used. In this case, only a radiation field in a direction of acquiring image data can be defined. For determining an irradiation direction, axial images are used. However, when some organs must not be exposed to radiation, a radiation field must be defined for each axial image.
Furthermore, according to a conventional therapy planning procedure based on a scanogram and axial images, since (axial) images produced during previous scanning are used for therapy planning, a CT image rendering an appropriate portion of a lesion at which an isocenter should be set may not be included in the images produced during previous scanning. In this case, re-scanning is a must. It therefore takes too much time for therapy planning. In addition, a load to an operator or a patient increases.
Even if the CT image rendering an appropriate portion of a lesion is included in the images produced during previous scanning, the reconstruction center of the image is not always consistent with a position at which an isocenter should be set. Moreover, a slice thickness seldom agrees with the width of each leaf of a multileaf collimator. Thus, high-quality images rendering planes aligned with radiation paths and assisting in therapy planning can hardly be provided.